In our earlier applications it was explained that in the production of continuous or semi-continuous materials by either a rolling extruding or other forming processes, for example the manufacture of steel bar in a rolling mill, it is usually desired to obtain data on the cross-sectional shape and dimensions of that product at least at the finishing end of the process. The invention disclosed in our earlier application provides goods results. However, the purpose of this invention is to improve the accuracy of the obtained data.